The accident happened during the morning commute. The commuter train came in too fast and slammed into a barrier at the end of the platform.
The train was packed with people standing between the seats, and the impact threw many of the passengers into each other and onto the floor.
It was Argentina's worst train accident in decades.
Emergency workers were slowly extracting dozens of people who were trapped inside the first car, said Alberto Crescenti, the city's emergency medical director.
The dead included 48 adults and one child.
The Associated Press contributed to this report.